The present invention relates to a method and apparatus for performing a continuous free-film deflection electrophoresis, in which a spatially narrowly constricted stream of a sample mixture which is to be fractionated is introduced into a thin strip of a buffer solution flowing through a separating chamber defined by two plane parallel walls, an electrical voltage is applied to the buffer strip to produce an electrical field therein which extends transversely to the direction of flow and parallel to the walls of the separating chamber, and the separated fractions are detected by an analyzer device disposed downstream of the sample introduction location, the velocity profile of the buffer stream between the separating chamber walls as well as electroosmosis effects as a result of the zeta potential of the separating chamber walls tending to impart a crescent-shaped deformation to the flow cross section of the fraction.
A publication by Allen Strickler and Terry Sacks in "Preparative Biochemistry", Volume 3, No. 3, (1973) pages 269-277, teaches that crescent-shaped deformations of the flow cross section of a given fraction, resulting from the velocity profile of the buffer stream, can be compensated by appropriate setting of the zeta potential of the separation chamber walls so that, for this fraction, the sample stream ribbon can be given a well defined cross section transverse to the flow direction, and thus optimum separation efficiency can be obtained. The setting of the zeta potential in the known arrangement is effected by applying an appropriate coating to the separating chamber walls, for example a pressure-sensitive polyester foil which, if required, may be provided with an additional layer of collodion, agar or gelatin.